In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Underground coal mining presents various safety issues, one of these being the prevention of fires or explosions occurring within the mine. These can occur due to the generation during mining of methane gas and coal dust (commonly known as mine dust), which can be trapped within the mine and is readily ignitable. Disadvantageously, the equipment used in coal mining can generate incendive sparks which can ignite such gas or dust. Therefore, mine operators attempt to minimise or eliminate the production of sparks underground.
Rotary cutting picks are employed in rotary cutters, which include a rotating drum carrying a plurality of picks. Excavation is performed by the picks biting into an earth face as they rotate with the drum. This highly aggressive engagement between the picks and the earth face can result in spark production between them.
Cutting picks employed for coal mining generally have a hard cemented tungsten carbide tip that is fixed, usually by brazing, to a steel shank. Picks of this kind are disclosed in various prior art, such as U.S. Pat. No. 6,113,195, EP 0274645 and DE 4226976. The tip of the picks can be either of the insert or cap style.
The insert style is shown in DE 4226976, in which a greater section of the axial length of the tip is anchored within a bore of the shank, than extends out of the bore. The cap style is shown in EP 0274645, in which the tip has a broader base than the insert style tip and the base is located and brazed into a relatively shallow recess in the forward end of the shank. The present invention is applicable to cap style picks.
In picks of the above kind, sparks can be produced between the tungsten carbide tip and the earth face and also between the steel shank and the earth face, although there typically is greater likelihood of spark production between the steel shank and the earth face.
The highly aggressive environment in which the picks operate also results in wear of parts of the pick that come into contact with the earth being mined. Typically the shank of a pick is formed of a material that is softer than the tip of the pick and therefore the shank is more likely to be subject to wear than the tip and this is particularly the case closer to the tip.
An annular sleeve, attached to the shank of the cutting pick adjacent the cutting tip has been proposed for spark reduction. See for example in Applicant's co-pending Australian patent application no. 2004201284. The present invention relates to a similar form of cutting pick, providing an arrangement to reduce the incidence of engagement of the sleeve with the earth face and/or with earth fragments dislodged during mining, in order to prolong the life of the sleeve against failure.